Panel loudspeakers operating according to the multi-resonance principle are known in the art and frequently referred to as “distributed mode loudspeakers.” These devices are essentially formed of a flat panel and at least one drive system, wherein oscillations are introduced in the panel by supplying low frequency electrical audio signals to the drive system. The drive systems for these devices are formed of one or several of electromagnetic drivers (shakers), depending on the application. However, the drive systems can also include piezo-electric bending oscillators, either alone or in combination with the aforedescribed shakers.
To properly operate panel loudspeakers, the loudspeakers are connected to a periphery using connecting elements. With this periphery, the entire panel loudspeaker can be secured from the outside and, on the other hand, the weight of the panel and of the drive system(s) can be supported in a manner advantageous for sound reproduction.
In sound reproduction systems implemented as panel loudspeakers, “bending wave radiation” can occur above a critical lower frequency limit, with the panel loudspeaker radiating the bending waves in a direction that depends on the sound frequency. A cross-section through a directional diagram shows a main lobe having a frequency dependent direction.
The panel of the panel loudspeaker consists of a sandwich structure, wherein preferably two opposing surfaces of a very light core layer are connected, for example by an adhesive bond, by way of a respective cover layer that is thin in comparison to the core layer. The panel loudspeaker has a particularly good sound reproduction if the material for the cover layer has a high dilatational wave velocity. Suitable material for cover layers are, for example, thin metal foils or fiber-reinforced plastic foils. The core layer also has to meet certain requirements and should have a particularly low density of, for example, 20 to 30 kg/m3). The core layer should also be able to withstand high shearing forces acting normal to the cover layers, which requires that the elasticity module in the direction normal to the cover layers is sufficiently large, whereas a small elasticity module parallel to the cover layers is acceptable. Accordingly, the core layer can be either anisotropic or isotropic. Suitable ultra-light core layer structures are, for example, honeycomb structures made of light metal alloys or resin-impregnated fiber-reinforced paper (anisotropic) and expanded foam (isotropic).
A system of the aforedescribed type can radiate sound waves by connecting the panel to a drive system which deforms the panel perpendicular to the plane of the cover layers in a wave-like pattern. The drive system can be a conventional magnet system that is attached to or integrated with the panel.
The efficiency of panel loudspeakers operating according to the multi-resonance principle can be optimized by leaving the marginal edge of the panel, if all possible, “unrestrained.” In other words, transverse oscillations propagating in the panel should be neither restricted nor attenuated in the marginal region of the panel.
Although the panel loudspeaker described above can successfully reproduce tones in the midrange and high-frequency range, it has been observed that low frequencies, i.e., bass tones, can only be faithfully reproduced by using panels having an undesirably large surface area. If the required large surface area is not provided, then the lowest panel frequencies which support the bass reproduction, move to the mid-frequency range.
It is a therefore an object of the invention to provide panel loudspeakers with relatively small panel surface areas that have an improved sound reproduction in the bass frequency range.